Method and apparatus for determining a material&#39;s characteristics by photoreflectance

ABSTRACT

A method and apparatus for determining the characteristics of materials, particularly of semi-conductors, semi-conductor heterostructures and semi-conductor interfaces by the use of photoreflectance, in which monochromatic light and modulated light beam reflected from the sample is detected to produce a d.c. signal and an a.c. signal, whereby the d.c. signal is applied to one input of a computer and the a.c. signal is used with another input of the computer which controls the light intensity of the monochromatic light impinging on the sample to maintain the d.c. signal substantially constant. A stepping motor is preferably utilized for varying the light intensity of the monochromatic light. Additionally, the modulation frequency of the modulated beam and/or the wavelength of the monochromatic light can also be varied by the computer. Growth conditions of semi-conductor materials as well as information about trap times can be obtained by analyzing the energy band gaps and determining the dependence of the in-phase photoreflectance signal on the pump modulating frequency, respectively.

FIELD OF INVENTION

The present invention relates to a method for determining thecharacteristics of materials, particularly of semiconductors,semiconductor heterostructures and semiconductor interfaces by the useof photoreflectance and to an apparatus for carrying out the method.

BACKGROUND OF THE INVENTION

The importance to study and characterize semiconductors (bulk or thinfilm), semiconductor heterostructures (superlattices, quantum wells,heterojunctions) and semiconductor interfaces (Schottky barriers,metal-insulator-semiconductors, semiconductor-electrolyte,semiconductor-vacuum, etc.) assumes ever-greater significance,particularly as many of these semiconductors and semiconductormicrostructures are fabricated by modern thin-film techniques such asmolecular beam epitaxy (MBE), metal-organic chemical vapor deposition(MOCVD), etc.

The materials and interfaces grown by MBE and MOCVD as well as othermethods can be characterized by a variety of optical, electronic andstructural methods including photoluminescence, photoluminescenceexcitation spectroscopy, absorption spectroscopy, modulationspectroscopy, Raman and resonant Raman scattering, cyclotron resonance,Hall effect, transmission electron microscopy, etc. Each of these toolsprovides specific information about the material of interest. Forcharacterization purposes the experimental tools should be as simple andinformative as possible. Many of the methods mentioned above arespecialized and sometimes difficult to employ. For example, a numberthereof, such as photoluminescence, photoluminescence excitationspectroscopy, absorption, cyclotron resonance, generally requirecryogenic temperatures. Because of its simplicity and proven utility,photoreflectance has recently gained importance for the evaluation ofsemiconductor thin films and heterostructures.

The basic idea of modulation spectroscopy is a very general principle ofexperimental physics. Instead of directly measuring an optical spectrum,the derivative with respect to some parameter is evaluated. This caneasily be accomplished by modulating some parameter of the sample ormeasuring system in a periodic fashion and measuring the correspondingnormalized change in the optical properties. The former perturbation istermed "external" modulation and includes such parameters as electricfields (electromodulation), temperature (thermomodulation), stress(piezomodulation), etc. Changes in the measuring system itself, e.g.,the wavelength or polarization conditions can be modulated or the samplereflectance can be compared to a reference sample, are termed "internal"modulation.

In modulation spectroscopy uninteresting background structure iseliminated in favor of sharp lines corresponding to specific transitionsbetween energy levels in the semiconductors and semiconductormicrostructures. Also, weak features that may not have been seen in theabsolute spectra are enhanced. While it is difficult to calculate a fullreflectance (or transmittance) spectrum, it is possible to account forthe lineshape of localized spectral features of modulation spectroscopy.The ability to fit the lineshape is an important advantage of modulationspectroscopy. Lineshape fits yield accurate values of the semiconductorenergy gap as well as broadening parameter. In addition, since"external" modulation spectroscopy is the a.c. response of the system tothe modulating parameter, photoreflectance also provides information inthe other modulation variables such as phase, modulation frequency,modulation amplitude, modulation wavelength as will be discussed morefully hereinafter.

In photoreflectance, the built-in electric field of the materials ismodulated by the photo-injection of electron-hole pairs created by apump beam of wavelength λ_(p) which is chopped at frequency Ω_(m) ·¹⁻¹³Despite its utility, the mechanism of photoreflectance is not fullyunderstood although several experiments have indicated thatphotoreflectance is due to the modulation of the built-in electric fieldthrough a recombination of the minority species with charge in traps.Thus, by measuring the dependence of the photoreflectance signal onΩ_(m) it is possible to gain information about trap times with the useof photoreflectance.

Though the use of photoreflectance has been known for more than twentyyears, experimental difficulties experienced with the photoreflectancemethod in relation to other modulation methods lessened interest in thephotoreflectance. These difficulties included scattered light from thepump beam and photoluminescence from the sample. A report published in1985 on the photoreflectance results on semiconductor heterostructuresby Glembocki et al., Appl. Phys. Letts. 46, 970 (1985); Proceedings ofthe SPIE (SPIE, Bellingham, 1985 ) 524, 86 (1985) produced renewedinterest in the use of photoreflectance to study not only thesesemiconductor structures but also to study bulk (thin film) material. Animproved apparatus involving a new normalization procedure which waspublished in 1987, will be described hereinafter by reference to FIG. 1.The new normalization procedure involved in the apparatus of FIG. 1helped to solve some of the aforementioned photoreflectance problems,i.e., scattered light from the pump beam and photoluminescence from thesample.

The present invention is concerned with further improving the prior artapparatus to achieve improved signal-to-noise ratios, to furthereliminate problems encountered in the prior art apparatus and inparticular to utilize novel computerized procedures to gain additionalinformation on the characteristics of the materials examined.

Accordingly, it is an object of the present invention to provide animproved method and apparatus for determining the characteristics ofcertain materials by photoreflectance which avoid by simple means theshortcomings and drawbacks encountered with the prior art apparatus andmethods in the use thereof.

Another object of the present invention resides in a novel apparatuswhich assures improved signal-to-noise ratio.

A further object of the present invention resides in an apparatusutilizing photoreflectance for determining characteristics of certainmaterials which is simple to use, provides great versatility in theinformation which can be obtained and assures high reliability andaccuracy.

Still another object of the present invention resides in an apparatusutilizing conventional computer technologies to obtain information onadditional characteristics of the materials.

Another object of the present invention resides in a method fordetermining characteristics of certain materials, such as semiconductormaterials and semiconductor heterostructures, which is simple to use,reliable in its operation and accurate in the results obtainedtherewith.

Another object of the present invention resides in a method of utilizingphotoreflectance which permits to obtain such additional information astemperature, uniformity of the sample tested and composition of thealloy.

Still another object of the present invention resides in a method basedon photoreflectance which permits continuous monitoring in themanufacture of materials, such as semiconductor materials, thateliminates the shortcomings and drawbacks encountered with the prior artsystems.

A further object of the present invention resides in a method based onphotoreflectance which permits accurate quality control in themanufacture of semiconductor materials.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in connection with the accompanying drawing which shows, forpurposes of illustration only, one embodiment in accordance with thepresent invention, and wherein:

FIG. 1 is a schematic block diagram of a prior art apparatus utilizingphotoreflectance; and

FIG. 2 is a schematic block diagram of an apparatus in accordance withthe present invention which utilizes photoreflectance in combinationwith computer controls to increase accuracy and versatility of theequipment.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawing, and more particularly to FIG. 1, referencenumeral 10 designates an appropriate lamp source whose light passesthrough a monochromator 11, to be referred to hereinafter also as probemonochromator. The exit intensity of the monochromator 11 at thewavelength λ is focused onto a sample 12 by means of conventional lensesor mirrors (not shown). I_(o) (λ) is thereby the intensity of light fromthe probe source 10, 11 striking the sample 12. Electromodulation of thesample 12 is produced by photoexcitation of electron-hole pairs createdby a pump beam from a pump source 13. The pump beam can be a laser or amonochromator and is chopped by a conventional chopper 14 at a frequencyΩ_(m). The beam reflected from the sample 12 is again collected byconventional second lenses or mirrors (not shown) and is focused on adetector 15, such as a photomultiplier, photodiode, photoconductor, etc.As the lenses or mirrors are of conventional construction, they are notshown for the sake of simplicity. Although FIG. 1 as well as FIG. 2 tobe described more fully hereinafter show the configuration forreflectance, the experiment can also be readily modified fortransmission by placing the detector behind the sample. Accordingly, theterm photoreflectance (PR) is used in this application in a broad senseto encompass both reflectance and transmittance.

The output of the detector 15 contains two signals, i.e., the d.c.signal and the a.c. signal. The d.c. signal is applied to a servo unit16 which drives a motor 17 controlling a variable neutral density filter18 to achieve normalization as will be discussed more fully hereinafter.The a.c. signal from the detector is applied to a conventional lock-inamplifier 19 which also receives a reference signal Ω_(m) from thechopper 14. The desired signal ΔR/R contained in the output of thelock-in amplifier 19 is applied to a conventional recorder 20 for visualdisplay of the information as well as to a computer 21 to permitline-shape fitting.

The light striking the detector 15 contains two signals: the d.c. (oraverage value) is given by α(λ)I_(o) (λ)R(λ) [α(λ)I_(o) (λ)T(λ)], whereα(λ) is the optical response of the collecting lens (or mirror),R(λ)[T(λ)] is the d.c. reflectance (transmittance) of the material whilethe modulated value at frequency Ω_(m) is α(λ)I_(o) (λ)ΔR(λ)[α(λ)ΔT(λ)],where ΔR(λ)[ΔT(λ)] is the modulated reflectance (transmittance). Thea.c. signal from the detector 15, proportional to αI_(o) ΔR(αI_(o) ΔT)is measured by the lock-in amplifier 19. The a.c. and d.c. signals fromthe detector 15 are denoted as V_(ac) and V_(dc), respectively.

In order to evaluate the quantity of interest ΔR(λ)/R(λ)[ΔT(λ)/T(λ)] anormalization procedure must be used to eliminate the uninterestingcommon feature αI_(o). In FIG. 1, normalization is achieved by the useof the variable neutral density filter 18 connected to the servo system16, 17. The variable neutral density filter 18 is placed in the opticalpath between the probe monochromator 11 (or other probe source such as adye laser) and the sample 12. The d.c. signal from the detector 15(V_(dc)) is fed to the servo 16 which varies the variable neutraldensity filter 18 and hence α(λ)I_(o) (λ) in order to keep V_(dc) as aconstant. Thus, in this procedure, the operating conditions of theexperiment, i.e., detector amplification, instrumental resolution, etc.are kept constant. In previous modulation experiments normalization wasachieved by (a) either varying the operating conditions of the systemsuch as amplification of the detector, entrance slit of themonochromator to control I_(o) or (b) by electronically dividing V_(ac)by V_(dc).

As pointed out above in photoreflectance experiments, problems arecaused by (a) diffuse reflected light from the pump source and (b)photoluminescence produced by the pump light getting into the detector15. This latter problem is particularly acute at low temperatures(semiconductor and semiconductor structures) and for superlattices andquantum wells even at 77K and sometimes room temperature. A main goal ina photoreflectance measurement is therefore to eliminate diffusereflected light from the modulation source and/or photoluminescenceproduced by the intense pump light. Both of them may reach the detector15 and then produce a spurious signal in the lock-in amplifier 19. Afilter 22 in front of the detector 15 in FIG. 1 assists in reducing thediffused light from the pump source.

The d.c. output of the photodetector 15, V_(dc), can be written as:

    V.sub.dc =α(λ)I.sub.o (λ)R(λ)K(λ)A(λ)               (1)

where K(λ) is the detector response (including the response of a filterif it is used) and A(λ) is the amplification factor of the detector.

In photoreflectance, the a.c. output (V_(ac)) is given by:

    V.sub.ac =[α(λ)I.sub.o (λ)R(λ)K(λ)+α(λ.sub.sp)I.sub.sp (λ.sub.sp)K(λ.sub.sp)]A(λ)           (2)

where I_(sp) is the intensity of the spurious signal due to scatteringand/or photoreflectance and λ_(sp) is the wavelength of the spurioussignal. The second term in Equation (2) is generally missing from otherforms of modulation spectroscopy such as electroreflectance,thermoreflectance, etc. (although it may occur in piezomodulationbecause of vibrations).

In the normalization procedure of FIG. 1, the quantity V_(dc) is kept atsome constant value C by varying I_(o) (λ), the light intensity incidenton the sample 12, by means of the variable neutral density filter 18. Insuch a procedure, the amplification of the detector 15 is not changedand hence A(λ) =A, where A is a constant. Thus, we can write

    α(λ)I.sub.o (λ)=C/R(λ)K(λ)A (3)

Substituting Equation (3) into Equation (2) yields for S_(LIA) (=V_(ac)/V_(dc)), the normalized output signal from the lock-in amplifier 19,the term:

    S.sub.LIA =[CΔR(λ)/R(λ)]+[α(λ.sub.sp)I.sub.sp (λ.sub.sp)K(λ.sub.sp) A]                    (4)

Since α(λ_(sp)), I_(sp) (λ_(sp)), K(λ_(sp)) and A are all independent ofthe probe wavelength, the second term in Equation (4) is a constant. Ifthis second term is not too large in relation to CΔR/R, it is fairlysimple to subtract the spurious factor and recover the true signal ΔR/R.It has been found that if CΔR/R.tbd.0.01(αI_(sp) KA), the subtractioncan be readily accomplished. The subtraction of the spurious signal isimportant for proper operation of the photoreflectance apparatus.

FIG. 2 is a schematic block diagram of a photoreflectance apparatus inaccordance with the present invention. It differs from FIG. 1 in severalimportant aspects. The probe monochromator 51 is driven by step-motor 52which is controlled by the computer generally designated by referencenumeral 70 of any conventional construction, programmed by conventionaltechniques to achieve the various functions, as will be described morefully hereinafter. Differing from FIG. 1, the variable neutral densityfilter 58 is not driven by a servo-mechanism but by a step-motor 53which is also controlled by the computer 70. It has been found that thesignal-to-noise ratio can be improved by a factor of 10 using thestep-motor control of FIG. 2 instead of the servo system of FIG. 1. Theimprovement is probably attributable to the fact that the servo systeminvolves small instabilities which are eliminated by the step-motorarrangement in accordance with the present invention. In addition, thecomputer 70 also controls the frequency (Ω_(m)) of the modulator 54, forinstance, in the form of a conventional chopper modulating the pump beamemitted by the pump source 63. Furthermore, the lock-in amplifier 55 isa two-phase model of known construction which determines the in-phaseand out-phase components of the photoreflectance signal (relative to thepump beam). The use of the two-phase lock-in amplifier 55 is importantfor evaluating the photoreflectance signal as a function of Ω_(m) toyield information about trap states as will be discussed more fullyhereinafter. It has also been found that signals from different depthregions of a sample structure produce signals with different phases anddependence on Ω_(m) which can be sorted out by the two-phase lock-inamplifier 55 and the computer-controlled modulating frequency Ω_(m).

In the apparatus according to FIG. 2, the probe light produced by lamp50 in conjunction with the probe monochromator 51, which can be adjustedby step-motor 52 to vary the wavelength λ of the probe light, isdirected onto sample 62 by the use of a lens(es) or mirror(s),schematically indicated by lens 76. The pump beam produced by the pumpsource 63 in the form of a laser or other appropriate secondary lightsource is also directed onto the sample 62 after being modulated bymodulator 54 whose frequency Ω_(m) can be varied by computer 70 by wayof line 81. The light reflected (transmitted) from the sample 62 isagain directed onto detector 54 by a lens(es) or mirror(s),schematically indicated by lens 77. The detector 54 may be similar todetector 15 of FIG. 1 and a filter 22 may again be interposed in theoptical path between the sample 62 and detector 54. The a.c. signal inthe output of detector 54 is applied to the input of the two-phaselock-in amplifier 55 to which is also applied a reference signal (Ω_(m))from the modulator 54 by way of line 82 to provide information about themodulating frequency Ω_(m) for purposes which will be explainedhereinafter. The d.c. signal from detector 54 is applied to computer 70by way of line 83 which includes an A/D converter 66 to change theanalog signal from detector 54 into a digital signal for use by thecomputer 70.

One output of computer 70 contains the desired photoreflectance signalΔR/R which is applied to a user-friendly display, e.g the display screen(not shown) associated with the computer. Another output of computer 70controls the step-motor 52 to vary the probe-light wavelength λ, by wayof line 84. A further output of computer 70 controls the step-motor 53to vary the adjustment of the variable neutral-density filter 58 by wayof line 85, and still another output of computer 70 controls thefrequency Ω_(m) of the modulator 54 by way of line 81, all for purposesto be explained more fully hereinafter.

COMPUTER FUNCTlONS

The computer 70 of any known type and with sufficient memories isprogrammed to provide the following functions. Since the programminginvolves conventional programming techniques, as known to personsskilled in the art, a detailed description thereof is dispensed withherein for the sake of simplicity.

The software used with computer 70 can be divided into three generalfunctions which can be designated as (A) Control and Data Acquisition,(B) Data Analysis including lineshape fit of photoreflectance spectraand (C) Comparison of Relevant Parameters obtained from the DataAnalysis with Theoretical Models.

A. Control/Data Acquisition Component

The control/data acquisition component serves the following functions:

(1) It controls the step-motor 53 which, in turn, varies the variableneutral density filter 58 in order to keep V_(dc) as a constant fornormalization purposes.

(2) It controls the step-motor 52 which drives the probe monochromator51. Thus, the range of wavelength λ for a given experiment can be set bythe computer 70. Also, the computer 70 can control the step-motor 52 formultiple scans to accumulate a preset signal-to-noise ratio.

(3) The computer 70 records both the in-phase and out-phase componentsof the signal from the lock-in amplifier 55.

(4) The computer controls the chopping frequency Ω_(m). When measuringthe dependence of the in-phase signal on Ω_(m), the computer 70 controlsΩ_(m). In this case the amplification of the electronic system(detector, pre-amplifier, lock-in amplifier) may change with Ω_(m). Thecomputer software automatically corrects for this.

(5) The computer control of a given experiment is an important functionin the subtraction of the spurious signal αI_(sp) KA from the truephotoreflectance signal C(ΔR/R). This is accomplished in the followingmanner. At a given probe beam wavelength (λ), which the computer 70 setsby means of step-motor 52, the computer sets the variable neutraldensity filter 58 for maximum density. Thus, the constant C (and I_(o))in Equations (3) and (4) is made equal to zero. In this case S_(LIA) (λ)=αI_(sp) KA. This signal is analyzed by the computer 70 to determine theabsolute phase of the pump beam appearing at the input terminal of thelock-in amplifier 55. Then the computer sets the lock-in amplifier 55 tothe correct phase with respect to the optical pump and offsets thespurious signal by changing the zero setting of the lock-in amplifier55. In the case of large spurious signals, this procedure is repeated atseveral different probe beam wavelengths.

B. Data Analysis/Fit

This component performs the following functions:

(1) It interfaces with components A and C.

(2) It presents the data (photoreflectance spectrum) on the computerscreen in a user-friendly manner. For example, photon wavelength isconverted to photon energy. Different spectral regions of the data canbe presented in magnified form.

(3) It provides data transformation including Fast Fourier Transform,filtering and smoothing procedures to improve signal-to-noise ratio andderivatives and integrals to analyze signals.

(4) It provides lineshape fit to data to extract important parameterssuch as photon energy of spectral features, linewidth of spectralfeature, amplitude and phase. This is an important aspect of the entireprogram.

(5) It fits to the dependence of the in-phase photoreflectance signal onchopping the frequency (Ω_(m)). This fit can be used to obtaininformation about trap times.

(6) It provides vector analyses of the in-phase and out-phase componentsof a spectrum to distinguish signals from different depth regions of thesample.

(7) If Franz-Kaldysh oscillations are observed, it evaluates peakpositions of the Franz-Kaldysh oscillations. This can be used todetermine electric fields and sometimes carrier concentration.

C. Comparison of Relevant Parameters of Data with Theoretical Models

In software component B, various pieces of experimental information areobtained such as positions of energy gaps, peak positions ofFranz-Kaldysh oscillations, etc. In order to make this data useful, itmust be compared with various models to give the user information aboutthe semiconductor or semiconductor structure.

(1) In thin film or bulk alloy materials, the position of the energylevels can be used to evaluate alloy composition. For example, in theGa_(1-x) Al_(x) As the Al composition (x) can be determined.

(2) In thin film or bulk elemental or binary semiconductors (GaAs, Si,etc.) the position of the energy gap can be used to determine thetemperature of the material.

(3) From the evaluation of linewidth one can gain information aboutcrystal quality.

(4) Strains can be determined from shifts and splittings of peaks.

(5) If Franz-Kaldysh oscillations are observed, positions of peaks canbe used to evaluate the built-in electric field and in some casescarrier concentrations.

(6) From the dependence of the in-phase component on _(m) andtemperature T, one can evaluate activation energy of trap states.

(7) In semiconductor microstructures such as superlattices, quantumwells and multiple quantum wells a complex theoretical model can becompared to the experimentally determined energy gap to evaluate widthof quantum wells and barriers, barrier height and in the case oflattice-mismatched systems (InGaAs), the built-in strain.

There is frequently a strong interaction between this aspect of theprogram and the lineshape fit discussed in software component B-4 above.In semiconductor microstructures the spectrum is often very complex andhence only major peaks will be fitted by the lineshape fit of softwarecomponent B-4. This initial information is then fed into the theoreticalmodel of software component C-7 to determine where other smallerfeatures should be found. This information is then introduced intosoftware component B-4 to complete the lineshape fit of the minorspectral features.

NEW APPLICATIONS

The present invention provides the following new applications ofphotoreflectance.

A. In-Situ Monitoring of Growth Conditions for MBE and MOCVD

It has recently been found that photoreflectance can be performed onGaAs and Ga₀.82 Al₀.18 As at temperatures up to 600° C. Thesetemperatures correspond to growth conditions for MBE and MOCVD. Thus byusing photoreflectance the temperature of the GaAs substrate can bemeasured in a contactless manner to about ±10° C. to within a depth ofonly a few thousand Angstroms, i.e., near the growth surface. Also, insitu monitoring of the growth of epitaxial layers of GA_(1-x) Al_(x) Ascan be performed. In both cases topographical scans can be performed toevaluate uniformity. The energy band gaps of semiconductors arefunctions of various parameters such as temperature, alloy composition(in the case of alloy semiconductors such as Ga_(1-x) Al_(x) As),stress, etc. Thus by accurately measuring the position of the energy gapone can gain information about these quantities. For example, accuratedetermination of the band gap of GaAs can be used to determine itstemperature. Thus, this application of photoreflectance makes use of thesharp, derivative-like structure and the lineshape fit to accuratelydetermine the energy of the energy gap. At present the two maintechniques to evaluate substrate temperatures over a wide range areinfrared pyrometry and thermocouples in intimate contact with the backof a substrate holder. Both of these techniques have serious drawbacks.Thermocouples measure the temperature at the back of a relatively thickblock of Mo to which the substrate is mounted by suitable means whichmay vary considerably in thickness and distribution from run to run. Theactual surface temperature of the substrate is thus known to within onlyabout ±50° C. This problem becomes even more serious as the use ofIn-free substrate holders becomes more widespread because thethermocouple is no longer in contact with the wafer. Infrared pyrometryis useful only above about 450° C. The pyrometer must be constantlyrecalibrated since the view port may become coated. This recalibrationprocedure can itself become a source of error. It is also important topick a pyrometer with a narrow, short wavelength spectral window so thatthe temperature measured is that of the wafer and not that of the heaterfilaments behind it.

Substrate temperatures at only a single point can be evaluated by usingreflection electron diffraction to observe the transition temperature atwhich <100>GaAs surface reconstruction switches from As stabilized to Gastabilized. This is taken as a measure of the bulk congruent sublimationtemperature of GaAs, reported to be between 625° C. and 638° C.

Photoreflectance overcomes all of these problems since it directlymeasures the optical spectrum of the wafer to within several thousandangstroms from the surface. Also, it can readily be performed over awide temperature range including T<450° C. It is relatively immune toviewport coating since it is a normalized technique. In addition, thequality and composition of epilayers such as Ga_(1-x) Al_(x) As can bedetermined during actual growth procedures.

The use of photoreflectance for the monitoring of MOCVD is particularlyimportant. Since MOCVD is not an ultra-high vacuum (UHV) technique, manyof the in-situ characterization tools available for MBE (which is UHV)such as RHEED, Auger, etc. cannot be used in MOCVD. In fact, opticalmeans such as photoreflectance are the only way to gain in-situinformation about the properties of the materials grown by MOCVD. Theseoptical methods can be used under a variety of pressure conditions.

Such experiments depend crucially on the computer-controllednormalization and subtraction routine discussed above as well as on theability to smooth and fit the lineshape in order to extract accuratevalues of the energy gaps. Because the materials can be measured up to600° C., a lot of black-body radiation reaches the detector which mustbe subtracted from the true photoreflectance signal.

B. Electric Field Distributions At Semiconductor HeterostructiveInterfaces

Photoreflectance can also be used as a contactless method to studyelectric field distributions and trap times at semiconductorheterostructure interfaces. The technique can be applied even when theheterostructures are fabricated on insulating substrates where it is notpossible to use conventional methods such as capacitance-voltage anddeep-level transient spectroscopy measurements. The electric fielddistributions and trap times can be related to charges at theinterfaces. The knowledge of the properties of these charges is veryimportant for device applications.

Use is thereby made not only of the sharp, derivative-like features ofphotoreflectance but two new important aspects are also employed, i.e.,the use of different pump wavelengths (λ_(p)) and modulating frequency(Ω_(m)) dependence of the in-phase component of the photoreflectancesignal. This permits investigation, for example, of the photoreflectancespectra at 300K from an MBE grown Ga₀.83 Al₀₁.17 As/GaAs/GaAs(epilayer/buffer/substrate) heterostructure as a function of λ_(p)(8200Å-4200Å) and Ω_(m) (20 Hz-4000 Hz). The buffer is semi-insulatingGaAs. The sharp spectral features permit to observe the direct band gapsof the different parts of the structure, i.e., epilayer, buffer,substrate.

By using different λ_(p), carriers can be photo-excited in differentregions of the structure and hence these separate sections can beselectively modulated.

By measuring the dependence of the in-phase component of thephotoreflectance signal as a function of Ω_(m) it is possible todetermine trap times at the various interfaces. The in-phase aspect isof importance for the following reason. Heretofore trap times werededuced from the dependence of the magnitude of the photoreflectancesignal on Ω_(m). The magnitude of the signal is the sum of the squaresof the in-phase and out-phase signals. However, this quantity does notobey the principle of superposition if there are contributions fromtraps of different time constants. Thus, it is rigorously correct foronly a single trap state. For a multiple trap state it is necessary toevaluate the Ω_(m) dependence of the in-phase component (with respect tothe pump beam) in order to employ the superposition of different traptime contributions.

The computer control of the pump-chopping frequency Ω_(m) as well as theaccumulation of the in-phase (as well as out-phase) component of thesignal is highly important for the success of such an experiment.

C. Determination of Strain at the SiO₂ /Si Interface

Photoreflectance can also be used to determine the strain in the Si atthe SiO₂ /Si interface in structures prepared by thermal oxidation of Siand subjected to rapid thermal annealing.

Although GaAs and related materials such as GaAlAs are becoming moreimportant, the workhorse of the electronics industry is still Si. One ofthe main reasons why electronic devices are fabricated from Si isbecause it is relatively easy to form a stable, insulating layer on theSi, i.e., SiO₂ on Si. Thus, the properties of the SiO₂ /Si interface areof considerable importance from a practical point of view.

The band gaps of semiconductors are functions of various externallyapplied parameters such as temperature, stress, etc. Thus, by evaluatingthe shifts of the energy gaps of semiconductors with one of theseparameters, it is possible to gain information about the parameter.

In the SiO₂ /Si system, the photoreflectance spectrum from Si with noSiO₂ is determined (reference sample) and is then compared with thephotoreflectance spectrum of SiO₂ /Si prepared and processed bydifferent means. By comparing the shift of the photoreflectance peak ofthe Si in the SiO₂ /Si configuration with that of the reference Si, itis possible to deduce the stress that has been introduced by theformation of the SiO₂. Some of this stress can then be relieved by rapidthermal annealing. This is a very important aspect of making an actualSi device.

COMMERCIAL APPLICATIONS

The present invention is also of significance to commercial applicationsas contrasted to its use as a tool in research and development.

The future of electronic, photonic and opto-electronic devices dependson the manufacture of these complex devices by thin film techniques suchas MBE and MOCVD as well as on various processing procedures. Thesesystems are relatively complex and expensive. Thus yield is veryimportant. There is a great need to characterize the materials andstructures of these devices as well as to characterize variousprocessing steps to improve yield and performance.

As discussed above, photoreflectance can yield a great deal ofinformation about these systems. The apparatus in accordance with thepresent invention is relatively simple (and hence inexpensive), compact,easy to use and can readily be employed at room temperature.Photoreflectance gives more information per dollar of investment thanalmost any other characterization method. Many other characterizationmethods must be performed at cryogenic temperatures. In addition, sincephotoreflectance is contactless, it can be used before the samples areremoved from the MBE or MOCVD growth chamber or processing chamber. Thisgreatly reduces turn-around time for the operator of such a complexsystem to make adjustments.

Also, since photoreflectance is contactless and can be performed at 600°C. while the materials are actually being grown, it can be used tomonitor and control growth parameters. This is of particular importancein MOCVD.

The following table is a summary of the present capabilities of thephotoreflectance apparatus in accordance with the present invention.

Presently Known Capabilities of Photoreflectance Spectroscopy

I. Thin Film and Bulk Material

A. III-V and II-VI Binary Materials (GaAs, InP, CdTe)

1. Crystal Quality: Qualitative determination of crystal quality fromlineshape of direct band gap (E_(o)) as well as higher lying transitions(E₁,E₂). One can also evaluate the effects of process-induced damage aswell as annealing (laser, rapid thermal, etc.).

2. Carrier Concentration: In high quality material with a carrierconcentration of about 10¹⁵ -10¹⁷ cm⁻³ (n-type or p-type) it is possibleto observe Franz-Kaldysh oscillations. The period of these oscillationsyields the surface electric field and hence carrier concentration.

3. Passivation of Surface States: If the Franz-Kaldysh oscillations areobserved then the effects of passivation of the surface and subsequentreduction of surface electric field can be studied.

4. Topographical Variations in Carrier Concentration but not AbsoluteConcentration--In the event that Franz-Kaldysh oscillations are notobserved, it is still possible to get information in the low-fieldregime. In this region, the magnitude of the signal is proportional tonet carrier concentration. Thus variation can be observed by measuringchange in amplitude. This has been clearly demonstrated for contactelectroreflectance but has yet to be shown for photoreflectance. Itshould work but has to be conclusively demonstrated.

5. Lattice-Mismatched Strains-Can evaluate lattice mismatched strainssuch as GaAs/Si, ZnSe/GaAs, etc.

6. Deep Trap States--From the dependence of the photoreflectance signalon the modulation frequency and temperature one can obtain trapactivation energies. Already some important preliminary results havebeen obtained but more systems need to be studied. It requires a rangeof temperatures up to 200°-300° C.

B. Alloy Semiconductor

1. Quantitive determination of stoichiometry of alloy materials such asGaAlAs, GaInAs, AlInAs, GaAlSb, HgCdTe, etc. For HgCdTe, it requires77K.

2. Crystal Quality--as explained above.

3. Deep Trap States--as explained above.

II. Heterostructures, Including Superlattices, Quantum Wells andMultiple Quantum Wells

A. GaAs/GaAlAs System

1. Fundamental quantum transition corresponding to lasing frequency.

2. Higher-lying quantum transitions to evaluate parameters such as welland barrier widths, barrier heights, etc. Yields completecharacterization of structure.

3. Two-Dimensional Electron Gas: One can observe signature of 2D-EG.

B. In GaAs/GaAs

1. Built-in strains.

2. Fundamental quantum transition, as explained above.

3. Characterization, as explained above.

C. GaSb/GaAlSb

1. Built-in strains

2. Fundamental quantum transitions, as explained above.

3. Characterization, as explained above.

While we have shown and described only one embodiment in accordance withthe present invention, it is understood that the same is not limitedthereto but is susceptible of numerous changes and modifications asknown to those skilled in the art, and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are encompassed by the scope ofthe appended claims.

We claim:
 1. An apparatus for determining characteristics of materials by photoreflectance, comprising monochromatic light source means, means for directing the monochromatic light onto a sample to be examined, pump beam means for directing a beam of energy onto the sample including modulation means for modulating said beam, means for directing at least a part of the non-absorbed monochromatic light and of the non-absorbed modulated beam from the sample onto a detector means operable to produce a d.c. signal and an a.c. signal in its outputs, computer means, means for applying the d.c. signal from the detector means to an input of the computer means, a lock-in amplifier means receiving at its input the a.c. signal from the detector means and operatively connected with its output to another input of the computer means, further means for controlling the modulation frequency of said modulation means from said computer means, and control means for keeping substantially constant the operating conditions in a given experiment by maintaining the d.c. signal constant at a predetermined value including variable means for varying the light intensity of the monochromatic light impinging on the sample and actuating means for controlling the variable means by an output from the computer means which is operable to cause the actuating means and therewith the variable means to move so as to achieve said predetermined value.
 2. An apparatus according to claim 1, wherein said variable means includes a variable neutral density filter, and wherein said actuating means includes a stepping motor controlled from the corresponding computer output.
 3. An apparatus according to claim 2, further comprising means for changing the wavelength of the monochromatic light by said computer means.
 4. An apparatus according to claim 3, wherein the means for changing the wavelength of the monochromatic light includes a stepping motor means controlled by said computer means and operable to change the wavelength of the monochromatic light.
 5. An apparatus according to claim 1, wherein the detector means receives the part of the monochromatic light and of the modulated beam reflected from the sample.
 6. An apparatus according to claim 1, wherein the detector means receives the part of the monochromatic light and of the modulated beam transmitted by the sample.
 7. An apparatus for determining characteristics of materials by photoreflectance, comprising monochromatic light source means, means for directing the monochromatic light onto a sample to be examined, pump beam means for directing a beam of energy onto the sample including modulation means for modulating said beam, means for directing at least a part of the non-absorbed monochromatic light and of the non-absorbed modulated beam from the sample onto a detector means operable to produce a d.c. signal and an a.c. signal in its outputs, computer means, means for applying the d.c. signal from the detector means to an input of the computer means including an A/D converter, a two-phases lock-in amplifier means receiving at its input the a.c. signal from the detector means and operatively connected with its output to another input of the computer means, further means for controlling the modulation frequency of said modulation means by said computer means, and control means for keeping substantially constant the operating conditions in a given experiment by maintaining the d.c. signal constant at a predetermined value including variable means for varying the light intensity of the monochromatic light impinging on the sample and actuating means for controlling the variable means by an output from the computer means which is operable to cause the actuating means and therewith the variable means to move so as to achieve said predetermined value.
 8. An apparatus according to claim 7, further comprising means for changing the wavelength of the monochromatic light by said computer means.
 9. An apparatus according to claim 8, wherein the means for changing the wavelength of the monochromatic light includes a stepping motor means controlled by said computer means and operable to change the wavelength of the monochromatic light.
 10. An apparatus according to claim 9, further comprising means for controlling the modulation frequency of said modulation means from said computer means.
 11. An apparatus according to claim 7, wherein said variable means includes a variable neutral density filter, and wherein said actuating means includes a stepping motor controlled from the corresponding computer output.
 12. A method for determining characteristics of a material, especially of semiconductors, semiconductor heterostructures and semiconductor interfaces by photoreflectance, comprising the steps ofa) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined, b) electromodulating the sample by directing onto the same a modulated pump beam from a pump source, c) collecting the light reflected from or transmitted by the sample in a detector which produces a d.c. signal and an a.c. signal which contains (i) a spurious signal caused by diffuse reflected light from the pump source and/or photoluminescence produced by the pump light reaching the detector and (ii) the true signal, d) normalizing the procedure by subtracting the spurious signal from the true signal and e) transforming the data to improve the signal-to-noise ratio.
 13. A method according to claim 12, wherein the data transformation includes Fast Fourier Transform, filtering and smoothing procedures.
 14. A method according to claim 13, wherein derivatives and integrals are produced from the data to facilitate signal analysis.
 15. A method according to claim 12, further comprising the step of displaying the data including the photoreflectance spectrum for further use.
 16. A method according to claim 15, wherein photon wavelength is converted into the photon energy and displayed on a computer screen.
 17. A method for determining characteristics of a material, especially of semiconductors, semiconductor heterostructures and semiconductor interfaces by photoreflectance, comprising the steps ofa) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined, b) electromodulating the sample by directing onto the same a modulated pump beam from a pump source, c) collecting the light reflected from or transmitted by the sample in a detector which produces a d.c. signal and an a.c. signal which contains (i) a spurious signal caused by diffuse reflected light from the pump source and/or photoluminescence produced by the pump light reaching the detector and (ii) the true signal, d) normalizing the procedure by subtracting the spurious signal from the true signal. wherein step (d) is accomplished by setting the intensity of monochromatic light to a minimum at a given wavelength of the monochromatic light, analyzing the then-obtained spurious signal to determine the absolute phase of the pump beam appearing at the input of a lock-in amplifier receiving the output of the detector, setting the lock-in amplifier to the correct phase with respect to the optical pump and offsetting the spurious signal by changing the zero setting of the lock-in amplifier.
 18. An apparatus according to claim 17, wherein the normalizing procedure is repeated at several different probe beam wavelengths in case of a large spurious signal.
 19. A method for monitoring in-situ the manufacture of materials such as semiconductors, semiconductor heterostructures and semiconductor interfaces made by thin film techniques, comprising the steps ofa) directing a monochromatic beam of probe light onto the material within its growth environment, b) directing a modulated pump beam onto the material, c) analyzing signals reflected from the material to determine its characteristics for use in controlling quality, yield and/or composition of the material being manufactured.
 20. A method according to claim 19, wherein the analysis by photoreflectance is carried out with the material located in its growth chamber.
 21. A method according to claim 20, wherein the analysis by photoreflectance is carried out under growth conditions of the sample.
 22. A method according to claim 19, wherein the in-situ monitoring of the material is carried out substantially continuously during at least parts of its manufacture.
 23. A method for determining growth condition of semiconductor materials, comprising the steps ofa) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined, b) contactlessly electromodulating the sample by directing onto the same a modulated pump beam from a pump source to therewith modulate the built-in electric field, c) collecting the light reflected from or transmitted by the sample with a detector which produces a d.c. signal and an a.c. signal which contains (i) a spurious signal caused by diffuse reflected light from the pump source and/or photoluminescence produced by the pump light reaching the detector and (ii) the true signal, d) carrying out a normalized procedure to recover the true signal by subtracting the spurious signal from the true signal, and e) analyzing the energy band gaps by measuring the position of a respective energy gap as determined under growth condition to obtain information regarding various parameters including temperature, alloy composition and stress.
 24. A method according to claim 23, wherein the normalizing step including the subtraction routine are carried out by computer control.
 25. A method according to claim 24, in which the energy of a respective energy gap is obtained by utilizing a sharp derivative-like structure and line-shape fit of the energy band gaps.
 26. A method according to claim 23, in which the temperature of a substrate near its growth surface is measured in a contactless manner.
 27. A method according to claim 26, wherein the substrate is a GaAs substrate.
 28. A method according to claim 27, wherein the temperature of the substrate is measured to within a depth of only a few thousand Angstroms.
 29. A method according to claim 26, in which uniformity is evaluated by topographical scans.
 30. A method according to claim 23, in which the growth condition of epitaxial layers of a semiconductor material is monitored in situ.
 31. A method according to claim 30, in which the semiconductor material is Ga_(1-x) Al_(x) As.
 32. A method according to claim 30, in which uniformity is evaluated by topographical scans.
 33. A method according to claim 23, in which the method is carried out at an elevated temperature up to temperatures corresponding to the MBE and MOCVD growth temperature of semiconductor materials.
 34. A method according to claim 23, in which step e) includes the steps of smoothing and fitting the line shape of the optical spectral lines.
 35. A method according to claim 23, in which uniformity is evaluated by topographical scans.
 36. A method for obtaining information about trap times, especially at interfaces of semiconductors and semiconductor heterostructures, comprising the steps ofa) directing a probe beam of monochromatic light onto a material sample whose characteristics are to be determined, b) electromodulating the sample with a frequency by directing onto the same a modulated pump beam from a pump source, c) collecting the light reflected from or transmitted by the sample in a detector which produces a d.c. signal and an a.c. signal containing an in-phase component, and d) determining the dependence of the in-phase photoreflectance signal on the pump modulating frequency to obtain information about trap times.
 37. A method according to claim 36, in which information about multiple trap states is obtainable from step d).
 38. A method according to claim 36, wherein the variation of the frequency of the pump beam modulation and the accumulation of the in-phase component of the signal are obtained by computer control.
 39. A method according to claim 38, wherein the frequency of the pump beam is varied by varying its pump chopping frequency. 